Crystalline propylene polymers have been widely used for a number of years as low cost thermoplastic resins in the production of fibers, coatings, containers, pipes, and the like. Propylene homopolymer is typically about 60 to 70 percent crystalline and consequently has good chemical resistance to hydrocarbons, alcohols, acids, and alkalies. To improve the toughness, however, propylene is commonly copolymerized with minor amounts of ethylene. The resulting decrease in crystallinity significantly lowers the melting point and heat resistance of the polymer. Fillers and reinforcements are often added to increase the stiffness and heat resistance and to reduce part shrinkage or warpage. The presence of such additives adversely affects the impact properties of the polymer. Thus, it is apparent there is a need for polypropylene-based resins having physical properties which are sufficiently improved such that they are suitable for use as engineering resins.
Propylene polymers have been blended with a variety of other polymers for the purpose of obtaining blends having properties superior to those of polypropylene. Since polypropylene is immiscible with most other polymers, however, it has typically been necessary to use a third polymer as a compatibilizer.
For example, U.S. Pat. Nos. 4,386,187 and 4,386,188 teach thermoformable polymer blend compositions comprising an olefin polymer (e.g., polypropylene), a styrene polymer, and a styrenic block copolymer rubber compatibilizer.
Bartlett et al (Mod. Plastics December 1981, p. 60) teach the use of hydrogenated styrene/butadiene block copolymers as compatibilizers for blends of general purpose polystyrene and polypropylene.
U.S. Pat. No. 4,582,871 discloses thermoplastic resin compositions comprising a propylene polymer, a styrene polymer, a styrenic block copolymer rubber compatibilizer, and an inorganic filler.
Jpn. Pat. No. 63-205341 teaches resin blends comprised of low molecular weight polypropylene, a styrene/maleic anhydride copolymer, and a styrenic block copolymer rubber compatibilizer.
Jpn. Pat. No. 59-66,444 discloses blends of polypropylene, an ethylene/glycidyl methacrylate/vinyl acetate terpolymer, and a graft polymer of ethylene-propylene-diene monomer rubber graft-polymerized with a mixture of styrene and acrylonitrile.
Jpn. Pat. No. 86-34037 teaches the modification of polypropylene with an epoxy group-containing olefinic copolymer such as an ethylene/glycidyl methacrylate copolymer.
U.S. Pat. No. 4,968,747 describes compatibilized blends of crystalline propylene polymers and styrenic copolymers wherein an epoxy group-containing copolymer is used as a compatibilizer.
Despite the advances which have been made in recent years with respect to blends of polypropylene and styrenic copolymers, it is apparent there exists a need for improved blends of this type whereby the compatibility of the blends is enhanced in order to simultaneously obtain better impact properties and heat resistance.